Fix ratio when using background images.

fix_ratio.js

function fix_ratio(ele){
  var img_ratio = 1.6;//ele.width()/ele.height(); 1920/1200
  var window_ratio = $(window).width()/$(window).height();
  if(img_ratio>window_ratio){
    ele.css("height","100%").css("width","auto");
  }else{
    ele.css("width","100%").css("height","auto");
  }
}

use with this css

.bg {
  background-position: 50% 0% !important;
  width: auto;
  height: auto;
  position: fixed;
  max-width: none !important;
  top: 0;
  left: 0;
  z-index: 0;
}